[plt-scheme] cptr?

Hello,

I'm working on a C extension that returns a cptr to scheme.
Scheme later uses the cptr in calls to the extension.

In C I can test if a scheme value is a cptr with SCHEME_CPTRP_TYPE,
but there doesn't seem to be a predicate in scheme to test if a value
is a cptr.  The obvious names "cptr?", "cptrp?", and "c-pointer?" are
undefined, but scheme does know that the value is a cptr.

  > x
  #<c-pointer:pad>

Is there a predicate for cptrness?

Thanks,

Ron Stanonik
stanonik at cogsci.ucsd.edu